Vacuum deposition of coatings using cathode sputtering induced by glow discharges is currently in widespread use. Sputter coating sources include cathode and anode structures, and are operated in an evacuated chamber backfilled with a sputter gas (typically argon at subatmospheric pressure). Positive ions formed in the space between anode and cathode impact a target located on the cathode surface, ejecting (by sputtering) atoms of target material from the surface and near subsurface atomic layers of the target. These sputtered atoms deposit on workpieces or substrates placed generally in line of sight of the target. Magnetron sputter coating sources employ magnetic fields crossed with electric fields in the vicinity of the target. The use of such magnetic fields can enhance glow discharge intensities and the attendant sputtering rates, extend operation to lower sputter gas pressures, confine the flow discharge to the neighborhood of the electrodes, and reduce electron bombardment of the substrates.
One type of magnetron sputter coating source in commercial use employs a nonmagnetic annular sputter target (cathode) of a generally inverted conical configuration surrounding an axially symmetric central anode. One example of such a sputter coating source may be found described in U.S. Pat. No. 4,100,055, issued July 11, 1978 to Robert M. Rainey and entitled "Target Profile for Sputtering Apparatus" and assigned to the assignee of the present invention. Magnetron sputter coating sources of the type just mentioned have been used extensively and effectively in important semiconductor wafer coating applications. In most cases, the materials being deposited are nonmagnetic, such as aluminum and its alloys, etc. In some cases, however, it has been desired to use the same sputter coating source to dispense such magnetic materials as iron, nickel, iron-nickel-alloys, etc., as well as the nonmagnetic materials for which the sputter coating source was initially designed. More recently, a need has emerged for coating magnetic disk substrates with multiple layers, at least one of which is of magnetic material. Magnetic disks are now vitally important in computer memory applications.
Simply replacing a nonmagnetic sputter target with a magnetic one of the same generally inverted conical configuration in the magnetron sputter coating source referred to above causes most of the magnetic field to be shunted through the magnetic target. This results in magnetic field intensities above the target which are too low to allow the desired magnetic enhancement of the glow discharge to take place.
In order to avoid excessive reduction in magnetic field intensities above the target, annular magnetic sputter targets of a generally L-shaped profile have been developed for use in the above-described sputter coating source. One such L-shaped magnetic sputter target is described in U.S. Pat. No. 4,060,470, issued Nov. 29, 1977 to Peter J. Clarke and entitled "Sputtering Apparatus and Method" (see FIG. 9). An essential feature of the L-shaped design is that the radial thickness of the outer annular band portion must be sufficiently thin that it is magnetically saturated in order that the magnetic field intensities above the target can be made sufficiently great that the desired magnetic enhancement of the glow discharge takes place. The higher the magnetic permeability and the saturation magnetization of the material, the thinner the annular band portion must be.
Magnetic sputter targets having an L-shaped profile contain much less material than nonmagnetic sputter targets of a generally inverted conical configuration. Moreover, the magnetic fields above the L-shaped magnetic targets lead to target erosion which is concentrated in the corner region. In relative terms, the inventory of magnetic target material usefully available for sputtering is therefore very limited.
It is also known that a magnetic material heated to or above its Curie temperature loses its ferromagnetism while so heated. Another approach to avoiding excessive reduction in magnetic field intensities above the sputter target, therefore, is to heat the target and maintain it at or above its Curie temperature. A disadvantage of this approach is that it requires means for monitoring the temperature of the target, coupled with a feedback system for achieving and maintaining the required Curie temperature. Also, the Curie temperature of some magnetic materials is so high as to be detrimental to the adjacent substrate being coated and/or to the vacuum seals for the system and/or to cause damage to the sputter coating source or target as a result of warping or excessive thermal expansion.
Most present-day magnetron sputter coating sources employ permanent magnets to provide the magnetic field required for glow discharge enhancement. As the sputter target erodes, the magnetic field intensities above the sputter target generally become stronger, leading to a lower electrical impedance of the glow discharge. This causes the sustainable voltage across the glow discharge to fall, bringing with it a decrease in sputtering yield. Maintaining a fixed sputtering rate, and hence a fixed coating rate, at a desired sputter gas pressure requires both higher current and a higher power. The glow discharge power supply must therefore be capable of providing extended ranges of voltage, current, and power, which adversely affects both power supply and power consumption costs.
Additional factors affect the electrical impedance of glow discharges. Sputter gas pressure is one. Others include thermal effects (expansion, contraction, and Curie-temperature-related) in sputter targets and magnetic circuits. The permanent magnet means used in most present-day magnetron sputter coating sources do not provide compensation of glow discharge impedance changes arising from such factors.
It is a well-known characteristic of glow discharges that the conditions for ignition (discharge initiation) and steady state operation are different. In some cases it is desirable to operate at a sputter gas pressure so low that ignition cannot occur with the magnetic fields in the sputter source (as provided by the usual permanent magnets) and the open-circuit voltage of the glow discharge power supply. One technique that can be used is to raise the sputter gas pressure sufficiently to allow ignition to occur, and then to reduce the sputter gas pressure to the desired operating level. Disadvantages of this approach include the relatively long time constants associated with the required pressure changes, plus the costs and complexity associated with controlling sufficiently quickly (that is, in times short in comparison with a coating cycle) the sputter gas pressure, which is normally controlled by flow rate and pumping speed.
An ever-present problem in magnetron sputter coating sources is sputter target cooling. In normal operation, much of the glow discharge power is dissipated in the target. In the source described in above-mentioned U.S. Pat. No. 4,100,055, a cooling jacket surrounds the outer circumference of the target. Under conditions of normal operation, the target expands into tight contact with the cooling jacket, whereupon heat flows from the sputter target into the cooling jacket. This arrangement works well for target materials with adequate thermal conductivities, such as various aluminum alloys and mixtures. For target materials having relatively low thermal conductivities, as do many of the magnetic materials, cooling by this method may be insufficient.
Another problem in magnetron sputter coating sources is that of predicting or determining the end of useful life. If an indirectly cooled sputter target is sputtered "through", sputtering of the target support member is likely to unacceptably contaminate the substrate being coated. If a directly cooled sputter target (that is, one in which the non-sputtered side is in contact with cooling water) is sputtered through, the entire associated vacuum chamber goes "up to water" (a situation to be assiduously avoided).
One method of dealing with the end-of-useful-life problem is to determine empirically how long (for example, how many kilowatt-hours of operation) a sputter target of a particular configuration and material is good for, to maintain an accurate log of accumulated kilowatt-hours, and then to simply replace targets in accordance with a rigid schedule. While this approach may be entirely appropriate for routine production using a mature process, alternative approaches may be more desirable in the early stages of process development. In particular, independent means of assessing the erosion, or life, status of a sputter target during process development would be highly desirable. In addition, such a means would provide a useful diagnostic tool even for routine production.
Yet another problem in the use of most magnetron sputter coating sources is that the distribution pattern of sputtered material arriving at the substrate being coated changes as the sputter target erodes during normal operation. With target erosion, the glow discharge moves into a region of higher magnetic field intensities. This causes the discharge to become increasingly concentrated, thereby producing an increasingly narrow groove as the erosion proceeds. This leads to changes in the distribution pattern of sputtered material (a "beaming" effect), and to a reduced inventory of usable sputter target material, with attendant reduced target life.
Accordingly, it is an object of the invention to provide a magnetron sputter coating source which can utilize efficiently a large inventory of magnetic sputter target material.
It is a further object of the invention to provide a magnetron sputter coating source which can utilize efficiently a large inventory of sputter target material, independent of magnetic permeability and saturation magnetization of the target material.
It is also an object of the invention to provide a magnetron sputter coating source in which the sputter target configuration can be independent of magnetic permeability and saturation magnetization of the target material.
It is another object of the invention to provide a magnetron sputter coating source in which a magnetic sputter target may be usefully operated at a temperature below the Curie temperature of the magnetic target material.
It is a further object of the invention to provide a magnetron sputter coating source in which a means is provided for controlling the electrical impedance of the glow discharge in the face of changes which may occur due to sputter target erosion.
It is yet another object of the invention to provide a magnetron sputter coating source in which a means is provided for controlling the electrical impedance of the glow discharge in the face of sputter target temperature changes.
It is a still further object of the invention to provide a magnetron sputter coating source in which a means is provided for controlling the electrical impedance of the glow discharge in the face of magnetic circuit changes.
It is another object of the invention to provide a magnetron sputter coating source in which a means is provided for controlling the electrical impedance of the glow discharge in the face of sputter gas pressure changes.
It is an additional object of the invention to provide a magnetron sputter coating source in which a means is provided for effecting glow discharge ignition at a desired steady-state sputter gas operating pressure.
Another object of the invention is to provide a magnetron sputter coating source in which an improved sputter target cooling means is provided.
An additional object of the invention is to provide a magnetron sputter coating source in which an independent means of assessing sputter target erosion is provided.
A further object of the invention is to provide a magnetron sputter coating source being a means for controlling the distribution pattern of sputtered material arriving at the substrate being coated, with such control being provided throughout the useful life of the sputter target.
A still further object of the invention is to provide a magnetron sputter coating source in which a means is provided for increasing the useful life the sputter target.
Additional objects and features will become apparent from the ensuing description of the invention.